General tips for gaining weight?

I'm 17M, 1.75m/5'9" and weigh around 55kg/121lbs (post porcelain throne, I checked). I've just been looking to get to a more healthy weight for my age and height. I've never weighed more than 60kg/132lbs in my life, so that seems like a nice first milestone.

I try to eat as much as possible right now, whilst keeping it healthy, but I live with my parents, so I can't always decide what I eat. I also don't do any exercises yet, because I really don't know what would initiate weight gain or weight loss, the latter I want to avoid.

Are there any small things to slowly get my weight up withouth having to eat specific things everyday?

Are there ant simple exercises you reccomend to gain weight?

Is there anything I should avoid, because it might cause weight loss?

'preciate it :)